Welding controllers are historically of two types: random fired non-synchronous and synchronous. Controls used on large welders are typically non-synchronous, and include an electromechanical cycle timer. Small welders are most often synchronous. These controls have been suitable in the past for producing welds of mild steel and performing other simple welds. However, for more complex welds, involving materials of dissimilar thickness, or of dissimilar or brittle materials, synchronous controls capable of complex power versus time profiles with tempering cycles are more often required.
The reason for timing welding events in synchronism with the power line may be described as follows. Welding times may be very short, perhaps only a few half cycles. Usually, the power switch used to control the welding transformer is made from inverse-parallel Silicon Controlled Rectifiers (SCRs) Whereas these SCRs may be turned on at any point in each half cycle of the AC power line wave, they turn off only when the current through them drops to zero, approximately the point when line voltage is zero. Thus, there may be an uncertainty of one-half cycle in determining the duration of the welding cycle. For example, a weld time of 0.025 seconds could be as short as three or as long as four half cycles depending on whether the initiation of the weld occurs shortly after, or at the moment, of, a power line voltage zero crossing.
However, if the weld time can be made to start at precisely the same point on the AC power line wave every time, as with a synchronous controller, the weld time can be precisely repeated at the desired number of cycles for every weld. This gives a very consistent weld joint.
Various types of welding controls are in current use. These include analog and digital controls based on discrete semiconductors, integrated circuits, and more recently, microprocessors. All of these controls use a large number of discrete components. As a result, they lack versatility, are difficult to assemble, subject to incorrect assembly, and are difficult to repair. The heat or power control on analog controls is usually an analog dial, making precise setting, as well as repeatability, difficult to accomplish.
Controls using digital integrated circuits are an improvement. These controls have found wide use because of the availability of complex IC's incorporating the equivalent of thousands of discrete semiconductors. However, these controls still require many IC's. Variations in weld schedule requirements are difficult or impossible to accomplish because modifications must be made to the hardware.
More recently, welding controls using microprocessors have been developed. These have the benefit of reducing the number of IC's used, as well as allowing for variations in weld schedule to be programmed into the memory. A typical microprocessor-based control has a number of discrete components: a microprocessor (MPU), random access memory (RAM) to store intermediate date, read-only memory (ROM) for program storage, an oscillator, or clock, to determine when events will happen, and an input/output device to communicate with the outside world.
In addition to the above, there are several other IC's required in any control to perform display, timing, monitoring and feedback of system operating parameters and other functions. The IC count in a typical microprocessor based welding control, therefore, may be fifteen or more.